The 20-year-old has been absent since suffering a severe knee injury playing for Spain in November 2023, missing his country's Euro 2024 triumph this summer.
"For Gavi it's the next step, it's an important step, all the coaches, the whole club are happy, that's is the next step and he is on the bench," Flick told a news conference.
The tenacious midfielder burst into Barcelona's first team in the 2021-22 season and became Spain's youngest ever player at 17, but was soon usurped by team-mate Lamine Yamal.
Barcelona will also be boosted by the return of Dani Olmo and Fermin Lopez after their recent injuries, while Flick confirmed teenage star Yamal is fit after a minor hamstring issue.
"The rest and the treatments for him were really good, so thank you to the national team," said Flick. "He is ready to play."
Flick said Dutch midfielder Frenkie de Jong, who also recently made his comeback from an ankle injury, felt discomfort in training and was a doubt for the game on Sunday at the club's temporary Olympic stadium home.
Barcelona still have several players out injured including Ronald Araujo, Ferran Torres and Andreas Christensen.